Fuel injection valve for internal combustion engines and a method for hardening the said valve

ABSTRACT

A fuel injection valve for internal combustion engines, having a valve body ( 1 ) and at least one injection opening ( 11 ) embodied in it, through which opening, controlled by a valve needle ( 5 ) that cooperates with a valve seat ( 9 ) embodied in the valve body ( 1 ), fuel can be injected into the combustion chamber of the engine. The valve body ( 1 ) comprises a high-alloy hot-work steel, which has been hardened by a case-hardening process (FIG. 1).

PRIOR ART

[0001] From the prior art, various methods for hardening steel are known. The intent is to vary the wear resistance and durability of the material as well as its workability. One example is known as case-hardening, in which carbon is incorporated into the layers near the surface of the workpiece. One method in this respect is described in U.S. Pat. No. 4,836,864, for instance. Other possibilities are nitriding of steels, in which nitrogen is incorporated into the layers near the surface of the workpiece. In fuel injection valves as well, of the kind used preferably for self-igniting internal combustion engines and described for instance in German Patent Disclosure DE 196 18 650 A1, the use of such hardened and treated steels for lengthening the service life of the steels is also known. In the course of further development in engines, the temperature stress on the fuel injection valve and thus on the needle seat in the valve body will continue to increase because of an increase in performance or an enhancement in the braking power, especially in utility vehicles. The case-hardened steels used until now, and the hardening methods employed for them, are no longer sufficient for these applications.

ADVANTAGES OF THE INVENTION

[0002] The fuel injection valve for internal combustion engines according to the invention as generically defined by the preamble to claim 1 has the advantage over the prior art that the valve body is shape- and wear-resistant up to high temperatures and is thus suited for use at all operating points of an internal combustion engine. The valve body of the fuel injection valve comprises a high-alloy hot-work steel that has been hardened by a case-hardening process. Combining the high-alloy hot-work steel with a suitable case-hardening process favorably combines the advantages of both the material and the hardening process. A marked increase in the vibration strength of the high-alloy steel from a reduced notch effect in use, a reduction in erosion in the ensuing grinding machining at the function geometries, and a reduction in the necessary initial hardness of the valve body and hence improved machinability, as well as a reduction in the vulnerability to cavitation in the valve body, particularly in the region of the valve seat, are obtained.

[0003] In an advantageous feature of the subject of the invention, the hot-work steel is shape- and wear-resistant up to a temperature of 450° C. As a result, the fuel injection valve is suitable for use at all possible operating points of the internal combustion engine.

[0004] In an advantageous feature of the invention, the high-alloy hot-work steel contains at least approximately 0.4% carbon, 5% chromium, 1% molybdenum, and trace amounts of other metal and nonmetal elements adding up to a total of less than 1%, and the missing proportion to make up 100% is iron. Such steels, such as X 40 CrMo V 51, are available on the market and can be used without further effort or expense.

[0005] In a further advantageous feature, the case-hardening process is a gas case-hardening process. As a result of the case-hardening, an otherwise necessary complicated postmachining operation becomes unnecessary.

[0006] The method according to the invention for hardening a valve body that is part of a fuel injection valve for internal combustion engines has the advantage that as a result of the treatment, the valve body has the requisite thermal strength for use in the combustion chamber of an internal combustion engine. To that end, the valve body is case-hardened in a gas atmosphere that contains a hydrocarbon and is then heat-treated at a temperature of approximately 900° C. in a vacuum, or at most at a pressure of 100 Pa. By the combination of these two method steps with a high-alloy hot-work steel, optimal hardening and wear resistance of the high-alloy hot-work steel can be achieved, so that this steel remains usable even at temperatures of the kind that occur under extreme loads in the combustion chamber of a self-igniting internal combustion engine.

[0007] In an advantageous feature of the method, the casehardening takes place at a pressure of less than 100 kPa. Because of this negative-pressure case-hardening process, a lessening in the formation of peripheral oxidations, which reduce strength, is obtained in particular.

DRAWING

[0008] In the drawing, a fuel injection valve is shown in longitudinal section as an example of a hardened valve body.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

[0009] The fuel injection valve shown in FIG. 1 has a valve body 1, in which a valve needle 5 is disposed longitudinally displaceably in a bore 3. A substantially conical valve seat 9 is embodied on the end of the bore 3 toward the combustion chamber, and at least one injection opening 11 is embodied in the valve seat and connects the bore 3 with the combustion chamber of the engine. The valve needle 5 has a guide portion 15, with which it is sealingly guided in a guiding portion 23 of the bore 3. In the direction of the valve seat 9, the valve needle 5 narrows, forming a pressure shoulder 13, and changes over into a shaft portion 17 of reduced diameter. On the end of that portion, a substantially conical valve sealing face 7 is embodied on the valve needle 5; the valve sealing face cooperates with the valve seat 9 and thus upon contact with the valve seat 9 closes the at least one injection opening 11 off from the bore 3.

[0010] At the level of the pressure shoulder 13, a pressure chamber 19 is embodied by a radial enlargement of the bore 3; it can be filled with fuel at high pressure via an inlet conduit 25. The pressure chamber 19 continues, toward the valve seat 9, in the form of an annular conduit 21, which surrounds the shaft portion 17 of the valve needle 3. In this way, the fuel flows out of the inlet conduit 25 through the pressure chamber 19 and the annular conduit 21 as far as the valve seat 9 and, if the valve sealing face 7 has lifted from the valve seat 9, through the injection openings 11 into the combustion chamber of the engine.

[0011] The valve needle 5 is controlled by the ratio of the hydraulic forces on the pressure shoulder 3 and the valve sealing face 7 on the one hand and a closing force on the other; this closing force acts on the end of the valve needle 5 remote from the combustion chamber and urges the valve needle 5 in the direction of the valve seat 9. One possible operating state of the fuel injection valve is that in which the closing force on the valve needle 5 remains constant, while the fuel pressure in the pressure chamber 19 and in the annular conduit 21 varies because of replenishing fuel from the inlet conduit 25. Because of the fuel pressure in the pressure chamber 19 and in the region of the valve seat 9, the valve needle 5 experiences a hydraulic force that is oriented away from the valve seat 9. If this hydraulic force is greater than the closing force on the valve needle 5, then the valve needle moves away from the valve seat 9 and thus lifts along with the valve sealing face 7 from the valve seat 9. If the pressure in the pressure chamber 19 falls below a certain threshold pressure, then the closing force on the valve needle 5 predominates, and the valve needle moves back in the direction of the valve seat 9, until the valve sealing face 7 closes the at least one injection opening 11 once again.

[0012] Because of the longitudinal motion of the valve needle 5 and the relatively hard impact of the valve needle 5 as it becomes seated on the valve seat 9, high forces on the valve body 1 are engendered in the region of the valve 9. Also because of the longitudinal motion of the valve needle 5 in the guiding portion 23 of the bore 3, there are friction losses between the valve needle 5 and the wall of the bore 3; if the material comprising the valve body 1 is soft, the result can be excessively high wear. To increase the hardness and thus the wear resistance, a so-called hot-work steel, which is among the tool steels, is used for the valve body 1. The use of high-alloy hot-work steels, such as the steel known as X 40 CrMoV 51, has proved to be especially advantageous.

[0013] This high-alloy hot-work steel can be exposed to operating temperatures of up to 450° C. without losing hardness and hence wear resistance. However, if the requisite quality demands for fuel injection valves are to be met, the surface of the valve body 1 must be additionally hardened. To that end, carbon is incorporated into the layers near the surface of the valve body 1 in a so-called case-hardening process, as a result of which the surface becomes hardenable. One possible case-hardening process is the gas case-hardening process, in which the steel is exposed, at a temperature of 900° C. to 1000° C., to an atmosphere comprising hydrocarbons and chemically inert gases, such as nitrogen (N₂). In this process, the carbon diffuses into the layers near the surface of the valve body 1, so that the carbon content increases there. The hardening depths here amount to from 0.3 to 4 mm. As a result of the case-hardening, the material becomes hardenable, and this hardening is done by ensuing heating in a vacuum furnace. In that process, the workpiece, in this case the valve body 1, is heated to approximately 800° C., and a substantial vacuum, or in case a pressure of less than 100 Pa, prevails in the hardening furnace.

[0014] The advantage of this method of hardening the valve body 1 is in the combination of a high-alloy hot-work steel with a gas case-hardening process that employs negative pressure, that is, a pressure of less than 100 kPa. As a result, the advantages of the hot-work steel are added together with those of the case-hardening and hardening method. A marked increase in the vibration strength of the high-alloy steel is obtained as a result of a reduced notch effect in the use of the negative case-hardening process, since peripheral oxidations are avoided. At the same time, a reduction in erosion in the ensuing grinding machining at the function geometries is obtained, since the injection opening 11 is postmachined by hydroerosive grinding.

[0015] Another advantage is the reduction in the necessary initial hardness of the fuel injection valve and thus improved machinability after the heat-treatment of the valve body 1. A reduction in the vulnerability to cavitation of the surfaces is also obtained, especially in the region of the inlet bore and needle seat of the valve body 1.

[0016] Besides the high-alloy hot-work steel X 40 CrMoV 51, still other high-alloy hot-work steels can also be used that have a carbon content of from 0.3 to 0.5%. 

1. A fuel injection valve for internal combustion engines, having a valve body (1) and at least one injection opening (11) embodied in it, through which opening, controlled by a valve needle (5) that cooperates with a valve seat (9) embodied in the valve body (1), fuel can be injected into the combustion chamber of the engine, and the valve body (1) comprises a steel, characterized in that the steel is a high-alloy hot-work steel, which has been hardened by a casehardening process.
 2. The fuel injection valve of claim 1, characterized in that the hot-work steel is shape- and wear-resistant up to a temperature of 450° C.
 3. The fuel injection valve of claim 2, characterized in that the hot-work steel contains at least approximately 0.4% carbon, 5% chromium, 1% molybdenum, and trace amounts of other metal and nonmetal elements adding up to a total of less than 1%, and the missing proportion to make up 100% is iron.
 4. The fuel injection valve of claim 1, characterized in that the case-hardening process is a gas case-hardening process.
 5. A method for hardening of valve body (1) that is part of a fuel injection valve for internal combustion engines and is made from a high-alloy hot-work steel, characterized by the following method steps: case-hardening of the valve body in a gas atmosphere that contains a hydrocarbon; heat-treatment of the valve body at a temperature of 900 to 1000° C., at a pressure of less than 100 Pa.
 6. The method of claim 7, characterized in that the casehardening takes place at a pressure of less than 100 kPa. 